1. Field of the Invention
The invention herein relates to the purification of gas. More particularly, it pertains to the purification of gases which are used in production of contamination-sensitive products.
2. Description of the Background
The circuitry on semiconductor chips is normally formed by photolithography. Each layer of the chip has a photoresist mask which defines the circuitry for that layer. Ultraviolet laser produced light is used to expose the photoresist and create the image of the circuitry on the chip layer.
The degree of definition of the circuit lines on the chip is a function of the wavelength of the laser light. As circuitry has become more complex, more and more circuit lines are being crowded into each unit area on the semiconductor layer. Not only does this mean that the lines themselves must be narrower, but it also means that their definition must be more precise to avoid either interference and short circuiting with neighboring lines or breaks (opens) in an individual line. The relationship between the ultraviolet light wavelength and the width of the circuit lines is direct: narrower circuit lines require shorter ultraviolet light wavelengths. Conversely, the shorter wavelengths of ultraviolet light have higher energies. Ultraviolet is generally defined as wavelengths between 400 nanometers (nm) and about 10 nm. Other regions of interest are Near Ultraviolet (NUV), which is approximately 400 to 300 nm; Deep Ultraviolet (DUV), which is approximately 300 to 100 nm; and Extreme Ultraviolet (EUV), which is approximately ≦100 nm. Currently commercial semiconductor production processes operate with 248 nm (KrF) and 193 nm (ArF) ultraviolet light wavelengths which allow production of circuit lines with widths of approximately 0.15 micrometers (μm). The industry currently projects that the line widths of 0.10 μm and smaller will become the standard shortly, which will require photolithography with ultraviolet light wavelengths of 157 nm.
Photolithography of semiconductor wafers is conducted in closed chambers with the laser lens and the target wafer being surrounded by a gaseous atmosphere. The gases used in process environments vary from compressed dry air (CDA) to inert gases. Common examples are CDA, He, N2, and He/O2 mixtures. Different manufacturers, processes, wavelengths of light, and other requirements favor different gases. As the field advances to lower wavelengths of light, i.e. more energetic light, inert gases will likely become dominant, especially N2 and He.
It is normal, however, for the gas itself to be contaminated with small amounts of reactive gases or vapors or particulate materials. All these contaminants can and do affect the production process in various ways. Molecular contaminants with absorbances in the UV range reduce the optical transmittance of the lithography tool. Residues, deposits, and condensates form on the optical components of the lithography tool which reduce light transmittance. The photoacids generated by photoresists during the lithography process are sensitive to quenching by molecular contaminants, especially alkaline contaminants like ammonia. In sub-248 nm lithography, these contaminants can be found in some or all of the components of the lithography tool.
A limited number of decontamination processes and products have been used in the past to produce “lens gases” of acceptable levels of purity. Most notable of these are carbon beds and particulate filters. As the required circuit line width and therefore the ultraviolet light wavelengths have decreased, however, decontamination processes which were once sufficient have become unacceptable because the degree of residual contamination has been found to be too high under the newer, more stringent requirements.
Metrology is the field within the semiconductor industry that is responsible for testing the wafer at various stages in the fabrication process. Wafers are tested by metrology devices throughout their manufacturing to ensure compliance and eliminate defective wafers. Metrology limits must always exceed the resolution limits of the lithographic processes. As device dimensions shrink metrology limits must follow suit, which will lead to greater contamination susceptibility in the metrology environment. Two technologies currently dominate the metrology market: optical metrology and electron beam metrology. Purification of the gases in the metrology environment is crucial to the accuracy of metrology.
For example, optical metrology devices may be within or outside the lithography tool itself. When the device is in the tool the metrology optics and laser beam become an integral part of the lithography tool. These systems are used for continuous monitoring of masks and reticles. The necessity of gas purification is the same as for the lithography tool environment.
In-line optical measurements are found outside the lithography environment. For example, ellipsometry tools are commonly used to measure thin-film uniformity after spin-on resist application. The photoresists in ellipsometers and interferometers are susceptible to molecular contamination from the environment.
CD-SEM is used as an inline metrology tool that has a high resolution. Although CD-SEM operation is conducted in a high vacuum chamber, the chamber is periodically opened to atmospheric contaminants, therefore, the purity of the purge gas is crucial in ensuring long-term tool stability. Additionally, the cassette(s) may be contained in a chemically purified environment to minimize their contamination. Furthermore, SEM reticle inspection systems may require purification to minimize damage to the reticle from molecular contaminants, especially when exotic materials become essential in future technology modes. Reticles may adsorb contaminants then desorb under process conditions.
The electron beam in CD-SEM produces charge contaminants in wafers. These occur when electrons are ejected from or adsorbed by the wafer surface. Therefore, another metrology tool must non-destructively measure the charge contamination after SEM and must also be contained in a decontaminated environment.
There are two kinds of contaminants that need to be considered in any purification process: those that are in the lens gas originally and those that get injected into the gas stream during the purification process. The contaminants commonly present in the lithography environment are water (H2O), hydrocarbons (HxCy) oxides of nitrogen (NOx), oxides of sulfur (SOx), ammonia (NH3), organic amines (R3N), metals (M+), halogenated and sulfonated hydrocarbons (RX, X=F, Cl, Br, I, SO3), siloxanes (SixHy), alcohols (ROH), sulfides (R2S), and halogen hydrides (HX, where X=F, Cl, Br or I), among others. Certain groups of contaminants present different problems in photolithography.    i. The group of neutral polar protic molecules—e.g., H2O and ROH—absorb radiation in the sub-248 nm range, condense easily on surfaces, and stabilize other polar contaminants by forming solvation complexes.    ii. The group of neutral polar aprotic molecules—e.g., NOx, SOx, R2S, and RX—absorb radiation in the sub-248 nm range and react with other contaminants to form harmful products. For example, the formation of ammonium sulfate residues on optical surfaces, known as optical hazing.    iii. The group of alkaline molecules, both protic and aprotic—e.g., amines, including ammonia—absorb in the sub-248 nm range; react with other contaminants; and quench the photoacid generated in the resist film. This quenching results in an undesirable T-shaped resist profile, known as T-topping.    iv. The group of acidic polar species, both Lewis (M+) and Bronsted (HX), absorb in the sub-248 nm range and react with other contaminants, mainly from Group (i), to form harmful acidic products. Bronsted acids on metal lead to loss of process control because the photoresists are based on specific acid content based on a timed release agent. Additional acid interferes with the control.    v. The group of neutral non-polar aprotic molecules—e.g., hydrocarbons and siloxanes—absorb in the sub-248 nm range, condense easily on surfaces, and are ubiquitous in the cleanroom environment. Under high energy conditions, hydrocarbons will leave a carbonaceous residue on the lens. Siloxanes can form an opaque layer of SiO2 on the lens.    vi. The group of environmental gases—e.g., CO2, and CO—absorb in the sub-248nm range and are ubiquitous in the cleanroom environment.
Contaminants also get injected into the gas stream during the purification process commonly from the decontamination material itself. For example, upstream carbon beds can release particulate contamination. Decontamination materials are normally high surface area solid materials either in granular form or as surface coatings on solid substrates. The flow of the gas being decontaminated or the process of surface adsorption can cause minute particulates to be generated from the decontamination material and entrained in the gas stream. Moreover, it is well known that the mixtures and concentrations of contaminants in gas streams vary from photolithography process to process and from gas vendor to gas vendor. Therefore, operators of individual photolithography processes must shop for different decontamination products depending on their specific process. Since most decontamination products have relatively narrow use limitations, in many cases a process operator cannot find an optimum decontamination product for the specific process and must compromise its decontamination specifications.
To eliminate contaminants from the lithography system, some or all of the components of the lithography tool are enclosed in sealed compartments. FIG. 1 demonstrates a typical lithography tool. The entire microlithography tool has not commonly been enclosed in its own chamber, but the trend toward improved molecular purity will require such isolation of the environment within the tool. Since the wafer must be placed in the laser path and removed from the tool, the environment must be constantly refreshed. Therefore, point-of-use purification of the lithography tool gas is necessary. In contrast, the laser is normally enclosed in a permanently sealed compartment. The laser is often sealed at its manufacturing facility, which is usually in a separate location from the tool assembly. The disadvantage to this practice is that it requires that a decontamination composition must also function for the lifetime of the laser. Therefore, purification of laser gas both prior to and during laser assembly, as well as point-of-use purification, is necessary.
The optics compartment is generally sealed from the rest of the tool environment, but it is often accessed by the operators, whereas the laser is not. Even if the entire lithography tool is contained in a purified environment, the optics portion will be further isolated, as it is not opened to the external environment as frequently as the lithography tool. Therefore, purification of the gases contained in the optics compartment, both at the manufacturing facility and point-of-use is necessary.
Typically, the gases present in the equipment are CDA, N2, He or Ar, and/or He/O2. Specifically, in the tool compartment and optics compartment, all of these gases may be present. In the laser compartment, N2 and He or Ar may be present. In scanning electron microscopy and other metrology tools, CDA and N2 are present. In the future, He or Ar may be used for these measuring tools.
Critical dimension (CD) quality and processing efficiency may also benefit from isolation and purification of the environments of various additional device uses in the semiconductor industry, e.g. bake ovens and metrology equipment. It may also become possible and beneficial to isolate the entire process of semiconductor manufacturing, from bare wafers to functional chips. Therefore, purification during assembly and point of use throughout the semiconductor industry is necessary.
It will therefore be evident that as the photolithography technology advances to lower ultraviolet light wavelengths and lower width microcircuitry lines the degree of decontamination of the lens gases must become greater, both for the removal of the original contaminants in the gas and also for the prevention of creation and injection of particulate contaminants into the gas during the decontamination process.
The Semiconductor Industry Association recommends that acceptable levels for all contaminants be in the 10–100 ppt (parts per trillion) range over the next few years. Problems have arisen in reaching that goal. Contaminant levels and decontamination requirements vary widely and depend on the gas, process, wavelength of light, and CD requirements. Other factors that increase the difficulty of reaching this goal include: the wide range of gaseous environments in different tools and different compartments within the tools, the wide range of possible contaminants and wide range of possible concentrations, the presence of non-atmospheric contaminants in combination with atmospheric contaminants, and the interference of certain contaminants with the attempted removal of other contaminants.
A single formulation capable of accomplishing sub-ppb levels for all contaminants does not presently exist. Some current purification technologies require energetic activation, either with heat or electrical stimulation, which may adversely affect the contamination levels by outgassing and by-product formation. Some purification technologies release particulate components that become entrained in the gas stream, especially at high pressure. Further, some purification technologies require periodic regeneration, replacement, and/or activation during the serviceable lifetime of the lithography tool. This causes an interruption in production and may increase the likelihood of outgassing, by-product formation, and/or particulate entraining.